1. Field of the Invention
The invention is related to transition metal nitrides and synthesis methods. The forms that the transition metal nitrides may take include thin film layers, micrometer-sized particles, and nanometer-sized particles. The applications include a thin film as a wear protective layer, particles for ultracapacitors, particles of catalysts, particles as an additive of a wear-resistant coating, and magnets.
2. Description of the Existing Technology and Some of its Problems
Transition metal nitride has been used as wear-resistant coatings and thermal barriers, due to its strong mechanical and thermal property. Along with the development of structural and mechanical engineering, the coatings and barriers are required to cover complicated and fine structures. In other words, the surface area relative to its volume is becoming larger in recent years.
It is also reported that transition metal nitride is useful as a functional material for ultracapacitors, catalysts, and magnets. To use transition metal nitride as ultracapacitor, catalysts, or magnets, it is important to increase surface area of the material. Nanotechnology using nano-sized particles has a potential of obtaining superior characteristics due to extremely large surface area relative to its weight. Functional materials requiring large surface area such as ultracapacitors, catalyst, and magnets receives tremendous benefit from nanotechnology.
With increasing demand of covering small-sized material with large surface area, the existing synthesis method is facing several challenges. Since transition metal is more readily oxidized than nitridized, synthesis of transition metal nitride requires elimination of oxygen and moisture. Synthesis methods typically involve vapor phase reaction in vacuum/air tight reactors. To form a nitride layer on transition metal parts, physical vapor deposition or plasma deposition are used. However, these methods are unable to coat complicated structure having deep blind holes because vapor phase reactant does not reach the bottom surface of the deep holes.
In the case of particle synthesis, vapor phase method is even less efficient because of extremely high surface area to cover. When the particles have size less than 10 nm or specific surface area larger than 10 m2/g, it becomes challenging for the gaseous agents to cover the entire surface. For example, vanadium nitride nanoparticles are synthesized using VCl4 as precursor. The VCl4 is dissolved and stirred in anhydrous chloroform inside a glovebox. The solution is then transferred to an Ar-filled glove bag, where the dissolved chloride is reacted with anhydrous ammonia gas over solution for 8 hours. The as-prepared powder is collected by evaporating the solvent at 100° C. under continuous NH3 gas flow. Final heat treatment for nitridization is conducted under an anhydrous ammonia atmosphere with a heating and cooling rate of 5° C./min. The temperature for heat treatment is 400° C. [1]. As shown in this example, the final heat treatment involves vapor phase reaction with constant ammonia flow at high temperature. A high temperature such as this can cause sintering, resulting in larger particle size than what would be achieved using a lower-temperature process.
The challenges in the existing synthesis methods of transition metal nitride are summarized as follows: (1) the existing methods use vapor phase reaction which is unable to cover surfaces of complicated structures or small particles; (2) the existing methods require constant flow of source gas such as ammonia or nitrogen; (3) some existing methods use metal halide precursors, which leaves halogen impurities unfavorable to ultracapacitor applications; (4) some existing methods requires multiple steps to obtain transition metal nitride; (5) some existing methods requires high temperature which causes larger particle size or unfavorable phase.